Support Champion skill idea: A buff that causes your target's attackers to be slowed

Watching some Warcraft 3, the Lich's Frost Armor has always been a very useful skill to invest into because it slows the target's attackers, making Blademasters a lot less lethal in trying to snipe your heroes. League's only real counterparts, IMO, to Frost Armor are Frozen Heart and Randuin's Omen, both which are passive and thus very weak in terms of their AS slow.

This leads to the topic at hand: What if there was a future Support Champion who could cast something akin to Frost Armor on a teammate, and then that teammate slows enemies who attack it?

I'd work it something like this:

Frost Shell Shields target ally for X seconds, absorbing incoming damage. While the shield holds, attacks have their attack and/or movement speed reduced for a short time.

Why have it tied to a shield? Counterplay. You can destroy the shield with spells and thus avoid the slow.

Good idea or bad idea?